Radius gimlet point anti-stripout screw

ABSTRACT

A self-tapping screw includes a head, a shaft, a tapered end and a rounded distal tip. A thread includes a thread start on the tapered end. The axial position of the thread start and a first full thread, the radius of the round distal tip and the diameter of the tapered end at the thread start are controlled in relation to the screw size for consistent, improved screw performance.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This present application claims benefit to U.S. Provisional Application No. 60/367,552, filed on Mar. 26, 2002.

FIELD OF THE INVENTION

[0002] The present invention generally relates to threaded fasteners, and more particularly, the invention relates to thread-forming or self-tapping screws.

BACKGROUND OF THE INVENTION

[0003] Thread-forming, self-tapping screws are known in the art for a variety of uses and applications. Unlike a standard screw that forms the male component which accepts a nut, or other female component of a threaded fastener system, a thread forming screw is driven into a body, forming threads therein as the screw advances. Depending on the size of the screw, the composition of the body in which it is driven and other factors, a pilot hole may be provided in the body before the screw is inserted.

[0004] One form of thread-forming screw, commonly referred to as a sheet metal screw, is provided for fastening together two relatively thin panels of material. Sheet metal screws have many applications and uses, including assembling panels and components of appliances, securing sections of ductwork, securing small parts of assemblies of various types, and the like. Some difficulties encountered in using sheet metal screws include providing sufficient holding force (resistance to stripout) with a lower driving force in relatively thin, sheet-like materials. Since the panels held together frequently are thin, a screw that is inserted through the panels often will protrude beyond the furthest surface from the first surface through which the screw is inserted. Screw ends protruding in this manner can damage materials and things coming in contact therewith, such as wires in appliances. The sharp, protruding points of known sheet metal screws can be a source of personal injury in the way of scratches and puncture wounds to persons coming in contact therewith.

[0005] Problems encountered in the design of sheet metal screws increase as the materials for which the screws are used become thinner, from efforts to reduce weight and decrease costs in the final assemblies, such as appliances. Thus, it is known to provide sheet metal screws of one type or design for use in very thin materials and of a different type or design for use in somewhat thicker materials. Even so, it is common that one screw will insert readily and quickly while a next similar screw used in the same materials will not “start” readily, but instead will dwell in the pilot hole with the threads riding on the surface of the material without advancing into the material. It is cumbersome and disadvantageous to have to obtain, store and use a variety of different sheet metal screws, depending upon the thickness of the material in which it is used. A single screw design useful in both thin and thick materials provides advantages to both the manufacturer and the user of the screw. Particularly on assembly lines, it is desirable that attachment sequences be performed consistently, and that all similar sheet metal screws inserted into similar materials react in the same way in starting and driving into the material.

[0006] It has been a long-accepted standard of the fastener art to provide sheet metal screws in standard sizes based on thickness. Numerical designations have been used for the different sizes, with the larger number being a thicker screw. Thus, by way of example, a #10 screw is thicker than a #6 screw, which is thicker than a #4 screw. It is also well known and universally accepted among screw manufacturers and users that different screw sizes are provided with different numbers of threads per inch of screw length. Again by way of example, it is commonly known and understood that a #4 screw has 24 threads per inch of shaft length, and a #10 screw has 16 threads per inch. This increases the complexity of manufacturing facilities and equipment needed to manufacture sheet metal screws in a variety of different sizes.

[0007] What is needed in the art is a standardized thread-forming tapping screw that overcomes some or all of the aforementioned disadvantages of known thread-forming tapping screws.

SUMMARY OF THE INVENTION

[0008] The present invention provides a thread-forming tapping screw useful in both thick and thin materials that has controlled thread start locations, common thread pitch among different size screws and a rounded screw tip.

[0009] In one aspect thereof, the present invention provides a self-tapping screw for screw sizes each having a standard diameter tolerance, each screw to be inserted in a pilot hole having a pilot hole diameter for each screw size. The screw has a head and a shaft extending from the head, the shaft defining a minor diameter of the screw. The shaft includes a tapered end having a distal tip. A helical thread is provided on the shaft. The thread includes a thread start disposed at a position on the tapered end an axial distance from the distal tip. The axial distance is related by a common thread start factor to a mean minor diameter determined from an average of the pilot hole diameter and the pilot hole diameter minus the tolerance for all the screw sizes.

[0010] In another aspect thereof, the present invention provides a self-tapping screw for screw sizes each having a standard diameter tolerance, each screw to be inserted in a pilot hole having a pilot hole diameter for each screw size. The screw has a head and a shaft extending from the head, the shaft defining a minor diameter of the screw. The shaft includes a tapered end having a distal tip. A helical thread is provided on the shaft. The thread includes a thread start disposed on the tapered end. The tapered end has a diameter at the thread start related by a common diameter factor to a mean minor diameter determined from an average of the pilot hole diameter and the pilot hole diameter minus the tolerance for all the screw sizes.

[0011] In still another aspect thereof, the present invention provides a self-tapping screw for screw sizes each having a standard diameter tolerance, each screw to be inserted in a pilot hole having a pilot hole diameter for each screw size. The screw has a head and a shaft extending from the head, the shaft defining a minor diameter of the screw. The shaft includes a tapered end having a distal tip. A helical thread is provided on the shaft. The thread includes a thread start disposed on the tapered end. The distal tip has a rounded shape formed on a radius related by a common radius factor to a mean minor diameter determined from an average of the pilot hole diameter and the pilot hole diameter minus the tolerance for all the screw sizes.

[0012] In a further aspect thereof, the present invention provides a self-tapping screw for screw sizes each having a standard diameter tolerance, each screw to be inserted in a pilot hole having a pilot hole diameter for each screw size. The screw has a head and a shaft extending from the head, the shaft defining a minor diameter of the screw. The shaft includes a tapered end having a distal tip. A helical thread is provided on the shaft. The thread has a first full thread on the shaft an axial distance from the distal tip related by a common distance factor to a mean minor diameter determined from an average of the pilot hole diameter and the pilot hole diameter minus the tolerance for all the screw sizes.

[0013] In a still further aspect thereof, the present invention provides a self-tapping screw for screw sizes each having a standard diameter tolerance, each screw to be inserted in a pilot hole having a pilot hole diameter for each screw size. The screw has a head and a shaft extending from the head, the shaft defining a minor diameter of the screw. The shaft includes a tapered end having a distal tip. A helical thread is provided on the shaft. The thread includes a thread start disposed at a position on the tapered end an axial distance from the distal tip. The axial distance is related by a common thread start factor to a mean minor diameter determined from an average of the pilot hole diameter and the pilot hole diameter minus the tolerance for all the screw sizes. The thread has a first full thread on the shaft an axial distance from the distal tip related to the mean minor diameter by a common distance factor for all the screw sizes. The tapered end has a diameter at the thread start related to the mean minor diameter by a common diameter factor for all the screw sizes. The distal tip has a rounded shape formed on a radius related to the mean minor diameter by a common radius factor for all the screw sizes.

[0014] An advantage of the present invention is providing a sheet metal screw that is easy to use and that performs consistently from screw to screw in both insertion and retention.

[0015] Another advantage of the present invention is providing a sheet metal screw that can be used advantageously for fastening into both thicker and thinner materials.

[0016] Still another advantage of the present invention is providing a sheet metal screw that has a common number of threads per inch of screw length for various screw sizes.

[0017] A further advantage of the present invention is providing a sheet metal screw that starts consistently from one screw to the next when used in similar materials.

[0018] A still further advantage of the present invention is providing a sheet metal screw that minimizes the risk of damage to things and injury to people coming in contact with the tip of the screw.

[0019] Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings, in which like numerals are used to designate like features.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is an elevational view of a thread-forming screw in accordance with the present invention;

[0021]FIG. 2 is a schematic representation illustrating various parameters of a thread-forming screw in accordance with the present invention; and

[0022]FIG. 3 is an end view of the screw shown in FIG. 1.

[0023] Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] Referring now more specifically to the drawings and to FIG. 1 in particular, a sheet metal screw 10 in accordance with the present invention is shown. Sheet metal screw 10 is of particular advantage when used for fastening together thin, sheet-like materials or panels, or for fastening objects and things to thin sheet materials.

[0025] Sheet metal screw 10 has a head 12 and a shank or shaft 14. A continuous thread 16 is provided on shaft 14, extending substantially the full length of shaft 14. Sheet metal screw 10 is a monolithic body, including head 12, shaft 14 and thread 16, that may be formed by various common and well known screw forming processes, including but not limited to stamping, rolling, casting and machining. Sheet metal screw 10 can be made from various materials, including but not limited to metals or plastics, the selection of which will depend upon the ultimate use for screw 10 and the materials of the body or panels (not shown) in which screw 10 will be used.

[0026] Head 12 includes a cap 18 and a flange 20. Cap 18 can be of various shapes and configurations for engagement with a driver used to insert screw 10 into the body or panels (not shown). Cap 18 can be designed for engagement by both power tools and manual tools. Thus, for example, cap 18 can be of hexagonal or other perimeter shape as shown, for engagement with a socket or other wrench type driver operating on the perimeter surface of cap 18. Further, cap 18 can be provided with a slot or slots (not shown), or other shaped opening (not shown) formed inwardly from an outer surface 22 of cap 18, to receive the blade of a screwdriver, hex key or other tool inserted into cap 18.

[0027] Flange 20 extends laterally from the juncture between cap 18 and shaft 14, and is of particular advantage when cap 18 is formed for engagement by wrench type tools operating on the perimeter surfaces thereof. Flange 20 forms a body against which a face of the tool can rest, for proper driving engagement between the tool and head 12. A thread-facing surface 24 of flange 20 can be provided with slots or ribs 26 or other surface treatment to achieve a desired surface-to-surface relationship between flange 20 and the body or panels (not shown) in which screw 10 is inserted. In some applications of and uses for screw 10, flange 20 is not required.

[0028] Without consideration of thread 16 integrally formed on the outer surface thereof, shaft 14 is a substantially cylindrically shaped body extending from head 12. Shaft 14 is of substantially consistent diameter throughout its length, except for a tapered end 28 forming a rounded distal tip 30.

[0029] Thread 16 extends helically around and along shaft 14, having a thread start 32 on tapered distal tip 30, and terminating near head 12. Thread 16 spirals from thread start 32 at a consistent thread pitch through out the length thereof, forming preferably 18 threads per axial inch of shaft 14 for all screws 10, regardless of the size of screw 10. Thus, in contrast to known sheet metal screw designs, in which screws of different widths are provided with different thread pitch, in accordance with a preferred embodiment of the present invention, thinner and thicker screws are all provided with 18 threads per axial inch of shaft 14, and manufacture is thereby simplified.

[0030] A thread angle, designated by numeral 34 in FIG. 1, is standardized at about fifty-eight degrees for all screw sizes in accordance with a preferred embodiment of the present invention.

[0031] Thread start 32 is located on tapered end 28 in a precise location relative to distal tip 30, as will be described in further detail hereinafter. Thread start 32 tapers outwardly from the surface of shaft 14, leading into a first full thread 36 on shaft 14.

[0032] With reference now to FIG. 2, various variable features of the present invention will be described in greater detail. For a greater understanding of the physical features and relationships of components in screw 10 according to the present invention, as shown in FIG. 2, the following references and definitions shall apply:

[0033] “A”—diameter of tapered end at the location of thread start 32.

[0034] “D_(min)”—mean minor diameter, calculated for the diameter of shaft 14.

[0035] “D_(maj)”—mean major diameter, calculated for the diameter of thread 16.

[0036] “F”—fade angle of thread start 32.

[0037] “L”—axial distance from thread start 32 to first full thread 36.

[0038] “P”—axial distance from distal tip 30 to first full thread 36.

[0039] “R”—radius of distal tip 30.

[0040] “S”—axial distance from distal tip 30 to thread start 32.

[0041] “T”—taper angle from shaft 16 to tip 30.

[0042] In accordance with the present invention, “A”, “P”, “R” and “S” are controlled to optimize the relationship between insertion torque requirements and stripout resistance for each screw size, and to provide consistent performance among similar screws 10 used in similar applications. The selections of values for “A”, “P”, “R” and “S”, to be described in detail hereafter, are dependent upon the diameter of shaft 14 and the diameter of threads 16. More specifically, the values for “A”, “P”, “R” and “S” are calculated from calculated values for the mean minor diameter (“D_(min)”) and the mean major diameter (“D_(maj)”) for each screw size. As mentioned above, regardless of the values of “D_(min)”, “D_(maj)”, “A”, “P”, “R” and “S”, all screws 10, regardless of screw size, in accordance with the present invention have a thread angle 34 of about fifty-eight degrees, and are provided with 18 threads per inch axial length of shaft 14. However, the values of “A”, “P”, “R” and “S” are selected for different screw sizes to achieve consistency in performance and improved performance over known sheet metal screw designs.

[0043] For any given screw size, the mean major diameter (“D_(maj)”) is the average of the minimum major diameter and the maximum major diameter. The minimum major diameter is calculated as the industry accepted standard maximum for a screw of that size (a known industry standard value for each screw size) minus 0.001 inch. The maximum major diameter is calculated as the minimum major diameter plus the industry accepted standard tolerance for a screw of that size (another known industry standard value for each screw size).

[0044] For any given screw size, the mean minor diameter (“D_(min)”) is the average of the maximum minor diameter and the minimum minor diameter. The maximum minor diameter is the industry accepted standard diameter for a pilot hole for a screw of that size. The minimum minor diameter is calculated as the maximum minor diameter minus the industry accepted standard tolerance for a screw of that size (still another known industry standard value for each screw size).

[0045] With “D_(maj)” and “D_(min)” determined as explained above, the values thereof are used for determining the values for “A”, “P”, “R” and “S”.

[0046] It is desirable that all screws 10 in a group of screws 10 start similarly in pilot holes of standard diameter for a screw of a given size. So-called thread pickup should be consistent from one screw 10 to a next screw 10, when both are installed in standard size pilot holes in similar material. This is particularly desirable on assembly lines, where screws are inserted rapidly and repetitively. The relationship between the position of thread start 32 and distal tip 30 should be consistent for all screws 10 of that size to achieve consistent start performance.

[0047] In accordance with the present invention, the axial distance “S” from distal tip 30 to thread start 32 on tapered end 28, for screws of all sizes, is between about four-tenths and about one-half of the calculated value of “D_(min)”. This common factor is referred to herein as the “thread start factor”. Preferably, for screws 10 of all sizes, “S”=0.451×“D_(min)”.

[0048] The radius “R” of distal tip 30 is between about two-tenths and about three-tenths of the calculated value for “D_(min)”, for each screw 10 in accordance with the present invention. This common factor for all screws 10 of the present invention is referred to herein as the “radius factor”. Preferably, for screws 10 of all sizes, “R”=0.287×“D_(min)”.

[0049] The diameter “A” of tapered end 28 at the location of thread start 32 is between about six-tenths and about seven-tenths of the calculated value of “D_(min)”, for each screw 10 in accordance with the present invention. This common factor for all screws 10 of the present invention is referred to herein as the “diameter factor”. Preferably, for screws 10 of all sizes, “A”=0.680×“D_(min)”.

[0050] In accordance with the present invention, for each screw 10, the axial distance “P” from distal tip 30 to the first full thread is between about 1.04 and about 1.05 times the calculated value of “D_(min)”. This common factor for all screws 10 of the present invention is referred to herein as the “distance factor”. Preferably, for screws 10 of all sizes “P”=1.048×“D_(min)”.

[0051] Taper angle “T” is held at thirty degrees (30°) for all screws 10. A thirty-degree taper angle provides more accurate control of “S” and “A”, and thereby the relative position of thread start 32 for improved starting performance of screws 10 in standard pilot holes.

[0052] Fade angle “F” occurs as a consequence of the calculated and controlled values of “R”, “A”, “S”, “T” and “L”.

[0053] Using the above calculations, examples of values for screws 10 of different sizes, in accordance with the present invention, are as follows: Size “D_(min)” “D_(maj)” “F⁰” “R” “L” “P” “S” “A” #4 .084 .1165 60.96 .024 .050 .088 .038 .057 #6 .1015 .1425 63.33 .029 .061 .107 .046 .069 #8 .122 .1680 60.51 .035 .073 .128 .055 .083 #10 .141 .1915 59.09 .040 .084 .148 .064 .096 #12 .1625 .2175 57.83 .047 .097 .170 .073 .111 #1/4 .1925 .2495 55.77 .055 .109 .193 .084 .134

[0054] Tests were performed comparing screws of the present design with standard sheet metal screws. A #6 screw of the present design and a standard #6 sheet metal screw having the standard 20 threads per inch were tested. Each was fastened through a 0.025″ thick bearing sheet having a 0.156″ diameter clearance hole, and a 0.025″ thick tapping sheet having a 0.105″ diameter tapping hole. Stripping torque for the #6-20 standard screw was 17.23 lb-in. and for the screw of the present design was 21.88 lb-in. The screw of the present invention exhibited an improvement of twenty-seven percent over the conventional screw.

[0055] Similar tests were performed comparing #8 size screws of the present design with standard #8 sheet metal screws having the standard 18 threads per inch of shaft length. Each was fastened through a 0.030″ thick bearing sheet having a 0.196″ diameter clearance hole, and a 0.030″ thick tapping sheet having a 0.125″ diameter tapping hole. Stripping torque for the #8-18 standard screw was 42.10 lb-in and for the screw of the present design was 43.24 lb-in. The screw of the present invention exhibited an improvement of three percent over the conventional screw.

[0056] Tests also were performed comparing #10 screws of the present design with standard #10 sheet metal screws having 16 threads per inch of shaft length. Each was fastened through a 0.030″ thick bearing sheet having a 0.220″ diameter clearance hole, and a 0.030″ thick tapping sheet having a 0.144″ diameter tapping hole. Stripping torque for the #10-16 standard screw was 41.00 lb-in and for the screw of the present design was 48.39 lb-in. The screw of the present invention exhibited an improvement of eighteen percent over the conventional screw.

[0057] The rounded distal tip 30, axial lengths from the tip to thread start 32 and to first full thread 36, as well as the diameters of the tapered end at various locations provide ease in centering and starting screws 10 in standard pilot holes. Further, the rounded distal tip 30 is blunt, reducing the potential for damage to things or injury to people coming in contact therewith. The standard thread pitch at 18 threads per inch provides equal or improved performance versus standard screws of different sizes provided with the standard thread pitches. Manufacturing is simplified. Since the screw can be used in both thicker and thinner materials, acquisition, storing and distributing the proper screw in manufacturing and assembly plants are simplified.

[0058] Variations and modifications of the foregoing are within the scope of the present invention. It is understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention. The claims are to be construed to include alternative embodiments to the extent permitted by the prior art.

[0059] Various features of the invention are set forth in the following claims. 

What is claimed is:
 1. A thread-forming tapping screw for screw sizes each having a standard diameter tolerance, said screw to be inserted in a pilot hole having a pilot hole diameter for each screw size, said screw comprising: a head; a shaft extending from said head, said shaft defining a minor diameter of said screw, and including a tapered end having a distal tip; and a helical thread on said shaft, said thread including a thread start disposed at a position on said tapered end an axial distance from said distal tip related by a common thread start factor to a mean minor diameter determined from an average of said pilot hole diameter and said pilot hole diameter minus said tolerance for all said screw sizes.
 2. The screw of claim 1, said thread start factor being between about 0.4 and about 0.5 for all said screw sizes.
 3. The screw of claim 1, said thread start factor being about 0.451 for all said screw sizes.
 4. The screw of claim 1, said tapered end having a diameter at said thread start related to said mean minor diameter by a common diameter factor for all said screw sizes.
 5. The screw of claim 1, said distal tip have a rounded shape formed on a radius related to said mean minor diameter by a common radius factor for all said screw sizes.
 6. The screw of claim 1, said thread having a first full thread on said shaft an axial distance from said distal tip related to said mean minor diameter by a common distance factor for all said screw sizes.
 7. The screw of claim 1, said thread provided at about 18 threads per inch length of said shaft for all said screw sizes.
 8. The screw of claim 1, said threads having a thread angle of about fifty-eight degrees for all said screw sizes.
 9. A thread-forming tapping screw for screw sizes each having a standard diameter tolerance, said screw to be inserted in a pilot hole having a pilot hole diameter for each screw size, said screw comprising: a head; a shaft extending from said head, said shaft defining a minor diameter of said screw, and including a tapered end having a distal tip; and a helical thread on said shaft, said thread including a thread start disposed on said tapered end, said tapered end having a diameter at said thread start related by a common diameter factor to a mean minor diameter determined from an average of said pilot hole diameter and said pilot hole diameter minus said tolerance for all said screw sizes.
 10. The screw of claim 9, said diameter factor being between about 0.6 and about 0.7 for all said screw sizes.
 11. The screw of claim 9, said diameter factor being about 0.68 for all said screw sizes.
 12. The screw of claim 9, said distal tip have a rounded shape formed on a radius related to said mean minor diameter by a common radius factor for all said screw sizes.
 13. The screw of claim 9, said thread having a first full thread on said shaft an axial distance from said distal tip related to said mean minor diameter by a common distance factor for all said screw sizes.
 14. The screw of claim 9, said thread provided at about 18 threads per inch length of said shaft for all said screw sizes.
 15. The screw of claim 9, said threads having a thread angle of about fifty-eight degrees for all said screw sizes.
 16. A thread-forming tapping screw for screw sizes each having a standard diameter tolerance, said screw to be inserted in a pilot hole having a pilot hole diameter for each screw size, said screw comprising: a head; a shaft extending from said head, said shaft defining a minor diameter of said screw, and including a tapered end having a distal tip; and a helical thread on said shaft, said thread including a thread start disposed on said tapered end, said distal tip have a rounded shape formed on a radius related by a common radius factor to a mean minor diameter determined from an average of said pilot hole diameter and said pilot hole diameter minus said tolerance for all said screw sizes.
 17. The screw of claim 16, said radius factor being between about 0.2 and about 0.3 for all said screw sizes.
 18. The screw of claim 16, said radius factor being about 0.287 for all said screw sizes.
 19. The screw of claim 16, said thread having a first full thread on said shaft an axial distance from said distal tip related to said mean minor diameter by a common distance factor for all said screw sizes.
 20. The screw of claim 16, said thread provided at about 18 threads per inch length of said shaft for all said screw sizes.
 21. The screw of claim 16, said threads having a thread angle of about fifty-eight degrees for all said screw sizes.
 22. A thread-forming tapping screw for screw sizes each having a standard diameter tolerance, said screw to be inserted in a pilot hole having a pilot hole diameter for each screw size, said screw comprising: a head; a shaft extending from said head, said shaft defining a minor diameter of said screw, and including a tapered end having a distal tip; and a helical thread on said shaft, said thread including a thread start disposed on said tapered end, said thread having a first full thread on said shaft an axial distance from said distal tip related by a common distance factor to a mean minor diameter determined from an average of said pilot hole diameter and said pilot hole diameter minus said tolerance for all said screw sizes.
 23. The screw of claim 22, said distance factor being between about 1.04 and about 1.05 for all said screw sizes.
 24. The screw of claim 22, said distance factor being about 1.048 for all said screw sizes.
 25. The screw of claim 22, said thread provided at about 18 threads per inch length of said shaft for all said screw sizes.
 26. The screw of claim 22, said threads having a thread angle of about fifty-eight degrees for all said screw sizes.
 27. A thread-forming tapping screw for screw sizes each having a standard diameter tolerance, said screw to be inserted in a pilot hole having a pilot hole diameter for each screw size, said screw comprising: a head; a shaft extending from said head, said shaft defining a minor diameter of said screw, and including a tapered end having a distal tip; a helical thread on said shaft; said thread including a thread start disposed at a position on said tapered end an axial distance from said distal tip related by a common thread start factor to a mean minor diameter determined from an average of said pilot hole diameter and said pilot hole diameter minus said tolerance for all said screw sizes; said thread having a first full thread on said shaft an axial distance from said distal tip related to said mean minor diameter by a common distance factor for all said screw sizes; said tapered end having a diameter at said thread start related to said mean minor diameter by a common diameter factor for all said screw sizes; and said distal tip have a rounded shape formed on a radius related to said mean minor diameter by a common radius factor for all said screw sizes.
 28. The screw of claim 27, said thread provided at about 18 threads per inch length of said shaft for all said screw sizes.
 29. The screw of claim 27, said threads having a thread angle of about fifty-eight degrees for all said screw sizes. 